The present invention relates to precision AC measurements and, in particular, to impedance measurements.
It is often desired to make both IV and CV (or LCR) measurements on a device under test (DUT). Typically, switching between IV and CV measurements on DUTs having at least three terminals (e.g., a MOSFET or BJT) has required either re-cabling at the DUT or locating switching circuitry close to the DUT. This is because two or more of the terminals are shorted together at, or near, the DUT to avoid high frequency reflections, while for IV measurements the terminals are typically not shorted.
Referring to FIG. 1, a typical prior art configuration for a CV measurement for a DUT 12 (4-terminal MOSFET) is shown. The capacitors 14, 16, 18, 20 are leakage capacitances to ground. A voltage 22 is applied to one side of the DUT 12. The shorted point 24 of the DUT 12 is connected to an auto balance bridge (ABB) 26. The ABB 26 forces the shorted point 24 to a virtual ground and measures the current 28 required to do so. Because the point 24 is at virtual ground, there is no potential across the capacitors 16, 18, 20 and thus no leakage current. The voltage 22 and the current 28 are then used to determine the impedance (Ohm's Law) across the DUT 12, independent of the leakage capacitances. It should be understood, that because of the duality of voltage and current, voltages and currents may be respectively interchanged and still produce the equivalent results.